In optics, all-fiber interferometers are important devices since they are useful in diverse applications; ultrahigh-resolution metrology and light modulation are just two examples. Fiber interferometers can also be key devices in modern instruments such as gyroscopes.
Numerous different approaches have been proposed to construct interferometers with conventional optical fibers.
The advent of the so-called micro-structured optical fibers (MOFs) opens many other alternatives to construct interferometers that can be very useful for wide application areas.
A micro-structured optical fiber (MOF), also known as photonic-crystal fiber (PCF), is a new class of optical fiber which has a cross-section (normally uniform along the fiber length) micro-structured from two or more materials, most commonly periodically arranged over much of the cross-section, usually as a cladding surrounding a core (or several cores) where light is confined. In fact, in literature, two types of MOF are known, depending on the physical mechanism that is responsible for confinement and guiding of optical light. One of these types of MOF is characterized in that the light is guided by total internal reflection (TIR). In the TIR case, the core is made of high refractive index material surrounded by a cladding made of a suitable arrangement of high and low refractive index regions, typically a pattern of microscopic air-holes in the transverse plane running along the whole fiber length. For this reason this kind of MOF is also named ‘holey fiber’.
Unlike conventional optical fibers, MOFs may be single mode from the visible to the near infrared. In addition, one can design MOFs with multiple cores or with air holes of different shapes (see “Photonic Crystal Fibers” by Philip Russell, SCIENCE, vol. 299, pp. 358-362, 17 Jan. 2003).
Novel modal interferometers have been constructed by cascading two identical long-period gratings (see “Mach-Zehnder interferometer formed in a photonic crystal fiber based on a pair of long-period gratings” by Jong H. Lim et al., OPTICS LETTERS, vol. 29, No. 4, pp. 346-348, 15 Feb. 2004) or by using a MOF with air holes of different diameters (see “Two-mode photonic crystal fibers” by J. Ju et al., OPTICS EXPRESS, vol. 13, No. 6, pp. 2082-2088, 21 Mar. 2005). These approaches suffer from several construction limitations, though.
In short, Mach-Zehnder interferometers based on long-period fiber grating pairs formed in conventional single-mode fibers have been widely studied. But, unlike in conventional single-mode fibers, it is rather difficult to form two identical 3-dB long-period fiber grating pairs in a PCF. The cladding mode of the single-mode fiber is completely understood but for a PCF the cladding mode is not well defined.
Mechanically inducing two identical long-period fiber gratings in the PCF, as Jong H. Lim et al. propose, in order to make an all-fiber Mach-Zehnder interferometers helps to study the properties of the cladding modes guided along the PCF, but it does not achieve to have them controlled and makes the modal guiding behavior restrictively dependent upon the severe construction requirements of the long-period fiber grating pairs in the PCF.
With regards to two-mode PCFs described by J. Ju et al., these fibers are characterized by: requiring critical conditions of injection and polarization of the light source into the fiber, having instability and strong dependency in temperature (as it is demonstrated in “Temperature sensitivity of a two-mode photonic crystal fiber interferometric sensor”, J. Jian et al., IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 18, No. 20, pp. 2168-2170, 15 Oct. 2006). In addition to these restrictions, excitation of the two modes is exclusively supported by a PCF constructed under very specific parameters: determined air-holes size and separation distance between air-holes.
Another way for construction of compact interferometers recently reported is by combining MOFs with tapering technology (as in “Compact modal interferometer built with tapered micro-structured optical fiber” by J. Villatoro et al., IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 18, No. 11, pp. 305-307, 1 Jun. 2006). The fiber is tapered by gently elongating it while a zone of length L0 is heated with a high temperature oscillating flame torch. With this “slow-and-hot” tapering process, a uniform-waist of width ρ0 is reached in the tapered MOF, in which the air-holes collapse and so is transformed into a solid unclad multimodal region. The fundamental HE11 mode of the holey fiber is coupled to the HE1m modes of the solid fiber. The contracting and expanding zones are equivalent to couplers in a fiber-optic Mach-Zehnder interferometer, while the modes of the solid section are equivalent to the arms. The improvement of this solution lies in the possibility of adjusting the geometrical parameters during fabrication of the interferometric device. Nonetheless, the disadvantage of this interferometer is an irregular and unpredictable oscillatory pattern.
All these techniques and others so far reported for interferometer fabrication based on MOFs present severe challenges in terms of complexity and/or manufacturability.